1. Field of the Invention
The present invention relates to the improved processing of energetic materials, in particular explosives, such as those used in warhead, munitions, and other highly energetic applications. More particularly, the method of this invention allows for the processing of explosives containing high solid loads of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0.5,903,11]-dodecane (also known and referred to herein as “CL-20” and “HNIW”).
2. Description of Related Art
For most existing weapons systems, one of the most critical ingredients, if not the most critical ingredient, in terms of explosive performance is the energetic filler. CL-20, with its substantial increase in performance output, is an organic oxidizing compound presenting significant opportunities in terms of energy capabilities for explosives. For example, the use of CL-20 as part of the explosive charge in weapons systems may provide, in comparison to conventional energetic fillers, increased antiarmor penetration and enhanced missile/torpedo effectiveness and lethality.
Many cast explosives are manufactured entirely in batch processes. A discussion of conventional batch processes for processing cast energetic materials is set forth in the background section of U.S. Patent No. 5,565,150. Generally, in a batch process, mixing is performed by first introducing liquid ingredients into an appropriate mixer, such as a standard vertical sigma-blade mixer, to form a plasticized binder. Examples of suitable liquid ingredients include polymeric binders, such as hydroxy-terminated polybutadiene, and plasticizers. Among the conventional plasticizers most commonly used with hydroxy-terminated polybutadiene are dioctyladipate (DOA) and isodecyl pelargonate (IDP). After the liquid ingredients have been mixed thoroughly, the solid ingredients are added in a multistep fashion, with an equal proportion typically being added in each step. Because of the high performance expected of modem explosive devices, cast explosive compositions typically contain high solid contents on the order of about 85 wt % to 92 wt %. Examples of solid ingredients include the following: organic energetic fillers such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane); inorganic oxidizers, such as ammonium nitrate, ammonium dinitramide, and ammonium perchlorate; and metal fuels, such as aluminum powder. Once the solid ingredients have been blended into the plasticized binder to achieve homogeneity, a curative (e.g., a diisocyanate) and a cure catalyst (e.g., triphenyl bismuth) are added to cure the plasticized binder. Prior to full cure, the mixture is cast, for example, into the case of a warhead or suitable mold.
When an effective amount of plasticizer is used in combination with RDX or HMX as the energetic filler, the solid ingredients are wetted and coated and the mixture takes on a sufficiently low viscosity to create a relatively free-flowing suspension that is castable without too much difficulty. Generally, it is possible to employ an effective amount of plasticizer to establish a free-flowing suspension without causing the plasticizer to exude from the cast explosive.
However, the inventor has found that the conventional plasticizers used for batch mixing do not create a free-flowing suspension when mixed with high solid loads of CL-20. Although CL-20 shares an association with RDX and HMX inasmuch as each of these compounds is categorized as a nitramine, in the context of this conventional processing technique, CL-20 does not share the same synergistic compatibility with conventional plasticizers as RDX or HMX. The addition of high proportions of CL-20 to conventional plasticized binders, such as hydroxy-terminated polybutadiene plasticized in DOA or IDP, produces a high viscosity formulation having poor flowability. As a consequence, expensive processing equipment and high power expenditures are required for stirring large batches of castable explosive compositions containing high CL-20 loads to create the shear forces needed to homogenize the composition. Even with high shear mixing equipment, it is often only possible to attain marginal homogeneity in the cast material. It is also difficult to cast the high viscosity explosive without generating air pockets in the cast explosive. The presence of air pockets in cast explosives is highly undesirable, since air pockets increase impact sensitivity and decrease performance. Attempts to lower the viscosity of the mixture and attain homogeneity in mixing by the addition of high proportions of plasticizer have also been unsuccessful. The amount of plasticizer needed to homogeneously disperse CL-20 loads in conventional plasticized binders and create a free-flowing casting mixture is typically so great that the plasticizer exudes from the cast explosive material. The exuding of plasticizer from the cast explosive material can cause the material to shrink and separate from its surrounding case.